CHAPTER 5 



Excretion 



The function of the excretory organs of animals is to maintain a 

 more or less constant 'internal environment'. To this end they may 

 be called upon to eliminate several classes of substances: (i) sub- 

 stances, like mineral salts or water, which are present in simple excess 

 in the diet ; (ii) the end products in the metabolism of organic nitro- 

 gen, sulphur and phosphorus, of which, particularly of nitrogen, 

 there is always a large surplus for excretion - these form perhaps the 

 most important category of excretory substances ; (iii) more or less 

 complex compounds, not uncommonly pigmented, which arise per- 

 haps as accidents or by-products in the course of other chemical 

 changes, and which have to be excreted because the chemical equip- 

 ment of the body is powerless to deal with them ; (iv) and, finally, in 

 order to maintain such properties of the blood as the hydrogen ion 

 concentration, or the osmotic pressure, the excretory organs may be 

 called upon to remove certain substances such as acids or bases, and 

 to hold back or reabsorb others such as water or specific ions. 



Excretion of dyes 



Organs may fairly be regarded as excretory if they perform these 

 functions, or eliminate one or more of these classes of substances. 

 But, in practice, it is not always easy to recognize these activities; 

 and therefore a second, experimental, criterion of excretory organs 

 has come to be used - the excretion of dyes introduced into the body. 

 This test must be used with discretion; for different dyes are ab- 

 sorbed by different tissues; methylene blue, for example, is taken up 

 by the nervous system, by the tracheal cells and by the oenocytes, 

 and is therefore of no use for this purpose. The most reliable dye is 

 indigocarmine, which seems to be removed specifically by the excre- 

 tory organs of all animals. When indigocarmine is injected into 



